The most prevalent signal structure for multi-carrier signal structures is Orthogonal Frequency Division Multiplex (OFDM), especially for new Wireless Fidelity (WiFi) and WiMAX applications. In OFDM, a data stream is converted from serial to parallel and each of the parallel data channels modulates a separate RF carrier, resulting in a relatively wide effective bandwidth and its attendant robustness to channel fades. The frequency spacing between the individual carriers is set equal to the data rate, such that each carrier coincides with nulls of all of the other carriers, thereby reducing inter-symbol interference. The OFDM composite signal is readily generated with the Inverse Fast Fourier Transform (IFFT), obviating the need to synthesize each of the RF carriers, and can be similarly demodulated at the receiver using the corresponding Fast Fourier Transform (FFT). This ease of use has been a major factor in the widespread use of OFDM.
Conventional methods of utilizing multi-carrier waveforms, such as the various forms of OFDM, do not result in constant envelope composite signals. Non-linearities in the power amplifiers used to transmit these signals result in signal distortions and performance degradation. As a result, current implementations use expensive, highly linear amplifiers in order to minimize these distortions, and the power is backed off considerably in order to maintain operation in the linear range, resulting in very poor power efficiency. Much of the ongoing research in this area is concerned with reducing the crest factor in the composite waveform and with development of adaptive pre-distortion techniques in order to mitigate the effect of amplifier non-linearities.